Today we woke up to reports of a huge unexploded World War II bomb being identified at a building site near Tower Bridge, in Southwark, south London.

With 1,200 households now evacuated while the Army manages the safe removal of the device, it highlights the importance of undertaking Unexploded Ordnance (UXO) assessments at the outset of any building development works, as the probability of identifying UXO originating from the Second World War might not be as uncommon as you think.

According to the Construction Industry Research and Information Association (CIRIA), between 2006 and 2009, approximately 15,000 devices were removed from construction sites – of which 5% were live.

Add to this, it has been reported that during WWII 17,000 tonnes of explosive fell on London alone, with figures suggesting that approximately 10% did not detonate on impact.

Site Assessments:

Help is available to the construction industry: the first port of call is a desktop tool called a Preliminary UXO Risk Assessment. It quickly provides a 'yes/no' answer as to whether there is any risk of encountering a bomb. It has been developed in line with guidance from CIRIA and endorsed by the Health and Safety Executive.

If a positive ‘yes’ output is identified, the next stage of assessment is to conduct a Detailed UXO Risk Assessment. This will detail the type of threat, the size, origin and also takes into account the proposed construction method and how that would impact on risk of detonation. Risk mitigation measures are then provided to allow construction to proceed.

The ‘Bomb Search’ assessments are available from Envirocheck, part of Landmark Information Group, and are provided by renowned experts in the field of UXO, 6 Alpha Associates.

“Encountering an unexploded bomb on a construction site is a low probability but extremely high consequence event, particularly when you take into account potential loss of life. Not to mention shockwaves which could spread underground for some distance, causing damage to foundations and other underground works. As we see it, it is the potential consequence, rather than the probability, which really drives the importance of undertaking detailed risk assessments before any work starts.”

Ultimately, at the start of any new ground works project, the first stage Preliminary search is a must. While the potential risk may be considered small, it is clear it should not be overlooked for the ultimate safety of all concerned.

Q: What type of Bomb is it?

The bomb appears to be a German WWII SC 250Kg UXB. It may contain up to 125Kg of high explosives. Although its fuzes (and it may contain one or two fuzes) will not function as designed in WII (they are likely to be electrical impact fuzes but might also be mechanical) such bombs often remain in a very dangerous state and as a result it is being be dealt with by Army Bomb Disposal Experts.

Q: The bomb is nearly 70 years old - why is it still dangerous?

The fuze(s) especially remain susceptible to shock and a strong knock (e.g. from a mechanical excavator or piling rig) especially if it is on or near the fuze pocket, it might have set the bomb-off.

Q: What might happen if it did explode?

Now that it has been discovered and partly unearthed, it is not now likely to explode spontaneously - but the work to render it safe is still highly dangerous. If during the Army render safe procedure it exploded (as is possible, although unlikely) the bomb will work very much as intended during WWII, i.e. a significant blast wave will be generated together with concurrent fragmentation of the bomb case, with shrapnel flying omni-directionally at supersonic speeds. A large crater will be generated at the seat of the explosion. Local buildings and infrastructure (e.g. underground services such as water and gas mains, as well as electricity and telecom cables) could be destroyed or badly damaged by underground seismic shock wave. Local foundations might be de-stabilised. Any LUL (Tube) asset in close proximity might be damaged. Anyone in very close proximity of the bomb itself (e.g. a Bomb Disposal Officer) would be killed instantly and those within the inner Police cordon that might be in direct line-of-sight of the bomb might also be fatally injured.

Q; How can we be sure that the general public are now safe?

The Army will have given the Police a safety cordon, which has apparently been set, initially, at 200m. Thousands of people have apparently been excavated for their own safety. The Army will also put in place blast reduction measures (e.g. by employing water blast reduction and/or importing thousands of tonnes of sand in order to ensure that if and when the bomb is rendered safe, any blast is contained).

Q: How often are UXB discovered in London and what is the Risk?

Whilst their discovery is a relatively low probability event in London (for example the London Fire Brigade state that WWII UXO is discovered in London once every couple of months), it is the prospective consequences (as outlined above) that actually drive the risk.

Q: What can and should be done to manage this risk?

In accordance with best UXO risk management practice (which was published by CIRIA on 2009), the following steps should be undertaken:

• Step 1 - A preliminary threat assessment should be undertaken in the form of a desk study, to establish if any bombs might be on the site. If there might be, then the next step should be taken;

• Step 2 - A detailed threat and risk assessment should be undertaken, also in the form of a desk study. This takes into account the types of underground activities that a developer might be planning (e.g. aggressive excavation and piling) and highlights not just the UXB threat but also the risks. If those risks cannot be tolerated and/or peoples' safety is put at risk, then the next step should be taken;

• Step 3 - A Risk Mitigation Design solution should be developed to reduce risks. This is commonly in the form of some type of UXB survey together with other on site measures deigned to look after ground workers;

• Step 4 - The execution of Site Survey and other measures, ahead of demolition, excavation or of piling activities;

• Step 5 - The delivery of a UXB Safety sign-off certificate to state that, as a result of steps 1-4, the risk has been reduce to As Low As Reasonably Practicable (ALARP) in accordance with the law and with best practice;

Q: Where can developers, contractors and ground workers get more advice?

Landmark and 6 Alpha Associates are the UK market leaders for the provision of UXB risk management advice and they provide all of the steps outlined above.

Q: Have 6 Alpha/Landmark Worked in this Region Before?

6 Alpha/Landmark have written several desk studies for this region of London. Subject to the type, extent and depth of construction/civil engineering work that may have been undertaken on the sites since WWII, and what is planned, most sites are generally classified as medium to high-risk in terms of UXB discovery.

Q: is this a problem that is confined to London?

No see the WWII bombing Blitz statistics for other cities:

CITY

NUMBER OF RAIDS

TONNAGE OF BOMBS

London

71

18000

Liverpool

8

1957

Birmingham

8

1852

Glasgow/Clydebank

5

1329

Plymouth

8

1228

Bristol

6

919

Coventry

2

818

Portsmouth

3

687

Southampton

4

647

Kingston-Upon-Hull

3

593

Manchester

3

578

Sheffield

1

355

Statistics for major cities during Blitz period of September 1940 to May 1941.